The bones and connective tissue of an adult human spinal column consist of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic and lumbar spine.
The spinal column of bones is highly complex in that it includes these more than 20 bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art that achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classifications suggest, lateral and anterior assemblies are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies. Posterior implants generally comprise pairs of rods, which are aligned along the axis along which the bones are to be disposed, and which are then attached to the spinal column by either hooks which couple to the lamina or attach to the transverse processes, or by screws which are inserted through the pedicles.
The pedicles are the strongest parts of the vertebrae and therefore provide a secure foundation for the screws to which the rods are to be attached. In order to obtain the most secure anchor for the pedicle screws, it is essential that the screws be threaded in alignment with the pedicle axis and not be allowed to deviate therefrom. Misalignment of the pedicle screws during insertion can cause the screw body or its threads to break through the vertebral cortex and be in danger of striking surrounding nerve roots. A variety of undesirable symptoms can easily arise when the screws make contact with nerves after breaking outside the pedicle cortex, including dropped foot, neurological lesions, sensory deficits, or pain.
Known surgical procedures to avoid misalignment of the pedicle screws involve recognizing landmarks along the spinal column for purposes of locating optimal tap hole entry points, approximating tap hole trajectories, and estimating proper tap hole depth. Some surgeons use a Kocher clamp applied to the vertebral bone for a reference mark and/or view radiographs or other medical images to better understand relative positions of the patient's anatomy. X-ray exposures and/or fluoroscopy can sometimes be used to monitor the advancement of a pedicle screws through the vertebra. Unfortunately, these procedures are subject to surgeon visual approximation errors, and anatomical landmarks are different for each patient. Further, prolonged radiation exposure to a patient is undesirable. U.S. Pat. No. 4,907,577 (Mar. 13, 1990) discloses a jig that is described therein as providing a safe route for drilling pedicle screw tap holes, by identifying a precise location for drilling to prevent deviation from the drilling direction so as to prevent injury during surgery to the nerve root or spinal cord. However, the jig has a variety of moving parts that must be adjusted and monitored simultaneously during the adjustments, making operation of the jig difficult and time consuming. Further, operation of the jig must occur during surgery, as it must be held adjacent the vertebral body to determine the proper adjustment settings. Finally, adjustment of the jig to the proper settings requires precise visual approximation by the surgeon, an activity that should be minimized to ensure that a misaligned trajectory is not established in place of a safe one.
More technologically advanced systems such as the StealthStation™ Treatment Guidance System, the FluoroNav™ Virtual Fluoroscopy System (both available from Medtronic Sofamor Danek), and related systems, seek to overcome the need for surgeons to approximate landmarks, angles, and trajectories, by assisting the surgeons in determining proper tap hole starting points, trajectories, and depths. However, these systems are extremely expensive, require significant training, are cumbersome in operation, are difficult to maintain, and are not cost effective for many hospitals.
U.S. Pat. No. 5,474,558 (Dec. 12, 1995) and U.S. Pat. No. 5,196,015 (Mar. 23, 1993) propose a procedure in which a screw opening is started in part of a skeletal region, e.g., a pedicle of a lumbar vertebra, and an electric potential of a certain magnitude is applied to the inner surface of the opening while the patient is observed for nervous reactions such as leg twitching. The opening continues to be formed while the electric potential is applied until a desired hole depth is obtained in the absence of nervous reaction to the potential. The direction in which the screw opening is being formed is changed to a direction other than the last direction, after observing patient reactions to the electric potential when the screw opening was being formed in the last direction. Unfortunately, this procedure is inherently reactive rather than proactive, in that the surgeon becomes aware of the misalignment after the patient exhibits a nervous reaction, and by that time the misaligned hole has been drilled.
Therefore, there is a need for a simple device that eases the difficulties associated with safely placing pedicle screws. Specifically, there is a need for such a device that assists a surgeon in making more accurate the surgeon's assessment of the proper insertion trajectory of the pedicle screw. Further, there is a need for such a device that does not require the surgeon to rely on visual approximations. In addition, there is a need for such a device that proactively determines the desirable drilling trajectory rather than reactively informing the surgeon when an improper trajectory has been used.